The present invention is concerned with reference electrodes, and the reference electrode portion of combination electrodes, which are employed to provide the stable reference potentials required by a variety of electroanalytical techniques, such as ion-selective electrode measurements, controlled potential coulometry, polarography, and the like. More particularly, the present invention is concerned with what commonly are referred to in the art as double junction reference electrodes.
A reference electrode most frequently is used in conjunction with an ion-selective electrode, either separately or in combination, to measure the activity (which is a function of concentration) of a given ion in a sample solution. Consequently, the discussion which follows primarily relates to such use. It is to be understood, however, that such discussion is not intended to in any way limit the spirit or scope of the present invention.
The two electrodes, i.e., the reference electrode and the ion-selective electrode, both of which are immersed in the sample solution, typically are connected to a means for measuring the potential difference between the electrodes, e.g., an electrometer. The reference electrode provides a constant electromotive force or potential against which the potential of the ion-selective electrode is compared. The latter potential consists of a constant component from the electrochemical half-cell of the ion-selective electrode and a variable component which is the potential across the sensing membrane and which is dependent upon the activity (concentration) of the ion being measured. The variable component, then, is readily correlated with ion activity (concentration) by known means. To give accurate results, the potential of the reference electrode should not change with the composition of the sample.
The reference electrode is designed to be minimally sensitive to changes in the external, sample ionic environment. It consists of at least three components: (1) a half-cell electrode (typically a silver-silver chloride mixture), (2) a half-cell electrolyte (typically 4 M potassium chloride solution saturated with silver ions), and (3) a reference junction. The half-cell electrode and half-cell electrolyte constitute an electrochemical half-cell having a known, stable, constant electrical potential. Direct physical, and therefore electrical, contact between the half-cell electrolyte and the sample solution is established through the reference junction which usually consists of a porous ceramic plug, metal or asbestos fiber bundle, sintered plastic, or like means of achieving a fluid mechanical leak.
As used herein, the term "half-cell electrode" means the solid-phase, electron-conducting contact with the half-cell electrolyte, at which contact the half-cell oxidation-reduction reaction occurs which establishes the stable potential between the half-cell electrolyte and the contact.
A major disadvantage of conventional reference electrodes of the above-described type is that the same electrolyte is used to accomplish two unrelated tasks: (1) setting the potential of the electrochemical half-cell, and (2) establishing contact with the sample solution via the reference junction. Half-cell ions, such as Ag.sup.+ in an Ag/AgCl electrode, Hg.sup.+ in a calomel electrode, and Tl.sup.+ in a thallium amalgam electrode, are also present at the reference junction where they may contaminate the measured solution and, in certain circumstances precipitate, clogging the junction.
For example, one of the major deficiencies of Ag/AgCl electrodes is the tendency of AgCl or other silver salts to precipitate within the junction, clogging it and interfering with free diffusion between the measured solution and internal electrolyte. Manifestations of a clogged reference junction include slow response, stirring-dependent potentials, and erroneous potentials at equilibrium.
Clogging increases response time by stopping the outward flow of junction electrolyte. In the absence of outward flow, the measured solution diffuses deep into the reference junction and temporarily serves as the junction electrolyte when the next solution is measured. The result may be a large diffusion potential which persists until the old sample is cleared from the junction by diffusion. With adequate outward flow, response time is minimized since the measured solution cannot penetrate deep into the junction and is flushed rapidly away during the next measurement.
Clogging by AgCl or other heavy metal salts may also cause non-ideal response in low ionic strength samples. This results in static error (due to the shift in potential upon entering the charged junction), stirring effects (due to shifts in static error with local changes in electrolyte concentration at the junction surface), and flow-dependent potentials (due to streaming potentials generated within the junction).
The tendency of the Ag/AgCl electrode to clog is particularly unfortunate, since it otherwise is an excellent electrode that offers high stability, ease of manufacture, low toxicity and extended temperature range.
AgCl tends to precipitate in the junction because AgCl is much more soluble in the usual 4 M KCl internal electrolyte than in the solutions in which the electrode is usually immersed. While the solubility of AgCl in pure water is very low, about 1.3.times.10.sup.-5 M, the solubility of AgCl in 4 M KCl is about 500-fold higher, around 7.times.10.sup.-3 M. This high solubility is attributable to the formation of negatively charged ionic complexes between Ag.sup.+ and Cl.sup.- having the general form Ag.sub.n Cl.sub.n+1.sup.-. When AgCl-saturated 4 M KCl flows or diffuses into a more dilute solution, the Cl.sup.- concentration is reduced and the excess silver chloride is precipitated. Precipitation of silver salt is often evident as a darkening of the external surface of the reference junction element and is particularly noticeable on older ceramic junctions.
My experiments indicate that junctions of conventional Ag/AgCl electrodes clog very rapidly. Even a new electrode can lose most of its flow capability after less than 24 hours in solution.
Contamination of the measured solution by heavy metal ions in the junction electrolyte is another problem associated with conventional reference electrodes. While silver is not particularly poisonous, its presence could be a problem in certain applications, e.g., photographic and forensic chemistry. Tl.sup.+ and Hg.sup.+ ions are very poisonous, and Hg.sup.+ has been observed to inhibit a variety of enzymatic reactions.
Metal salts cannot simply be omitted from the electrolyte of conventional reference electrodes since they are required to establish a stable electrode potential. Even if the metal salt is initially located only at the half-cell element, as in an AgCl-dipped silver wire, the salt will dissolve and quickly spread by diffusion and convection until the electrolyte is saturated. Deliberate confinement is required.
One approach to eliminating undesired ions in the junction electrolyte is to use a so-called "double junction electrode", in which separate compartments containing the reference-junction and half-cell electrolytes are connected by an internal liquid junction provided by, for example, a porous ceramic plug. Double junction electrodes are widely used in ion-selective electrode measurements where it is desirable to use a salt other than KCl as the junction electrolyte, but have not been generally used for the specific purpose of excluding heavy metal ions from the junction electrolyte. Nevertheless, they could be used for the latter purpose. However, conventional double junction electrodes have several related deficiencies. Double junction electrodes of current art have inner junctions of ceramic or other materials which are far too permeable to adequately inhibit flow under pressure without using thicknesses that needlessly increase the electrical impedance of the electrode. Stated differently, common junction materials are too permeable to yield barriers through which mixing is limited by ionic diffusion rather than liquid flow. In a typical double junction electrode, the inner half-cell compartment is refillable and the half-cell electrolyte flows under gravity through the inner junction into the junction electrolyte. This requires periodic refilling of the inner electrolyte and causes contamination of the external electrolyte. However, even if the inner compartment is sealed, mixing between inner and outer electrolytes can still occur as a consequence of diffusional interchange and also as a consequence of bulk flow through the inner junction due to pressure gradients brought about by thermal expansion or changes in ambient pressure.
In particular, inner junction materials of the current art are generally so permeable that, in sealed half-cell configurations, the mixing of half-cell and junction electrolytes caused by flow due to atmospheric pressure variations will far exceed the mixing caused by ionic inter-diffusion in the absence of flow. For example, the Corning double junction electrode (Cat. No. 476067, Corning Medical and Scientific Division, Corning Glass Works, Corning, N.Y.) uses a barely porous (1% void-volume) ceramic that was specifically developed for low flow. However, even with this ceramic, measurements of flow versus pressure indicated that solution exchange caused by average atmospheric fluctuations of .+-.3 cm Hg would be about ten-fold higher than exchange due to diffusion. Atmospheric fluctuations of .+-.3 cm Hg are equivalent to a steady head of about 1 cm in driving solution through the junction.
Thermal expansion and water vapor pressure at elevated temperatures (e.g., 90.degree. C.) were observed to cause much higher internal pressures and rapid loss of half-cell electrolyte. This expulsion of electrolyte from the half-cell compartment was accompanied by electrolyte dehydration, causing an increase in ionic concentrations and, consequently, a drift in the electrode potential. Also, half-cell dehydration was found to cause thermal hysteresis by causing the KCl concentration to rise high enough to exceed saturation levels at lower temperatures.
Some double junction electrodes have half-cell compartments containing electrolyte gelled with thickening agents such as water-soluble organic polymers. Such gelling of the inner electrolyte helps cut down flow, but I have found gelling agents to be relatively ineffective in preventing flow over the wide range of temperatures and pressures to which electrodes may be subjected. Also, the thickeners commonly used (agar and sodium carboxymethylcellulose) are subject to bacterial, thermal, and chemical degradation, and may also cause clogging of the inner junction. Since these thickeners also generally bear electrically charged groups, such clogging may cause erratic and drifting electrode potentials if the junction electrolyte is low in ionic strength.
Finally, the role of junction resistance in preventing diffusional exchange of ions has not been appreciated in the prior art. I have found that the rate of diffusional exchange of solution species through a porous barrier is inversely proportional to the electrical resistance of the barrier when saturated with a test electrolyte, but is independent of the barrier's size, shape, or structural detail. Thus, the effectiveness of a diffusion barrier is completely characterized by its electrical resistance value, which must usually be appreciable to adequately retard diffusional mixing of electrolytes. (The resistance value of the barrier may be measured by saturating it with 4 M KCl electrolyte, applying a voltage to electrolyte solutions separated by the barrier, and determining the ratio of the voltage across the barrier to the resulting ionic current.) It is noteworthy that in many prior art reference electrodes which provide separate compartments for half-cell and junction electrolytes, the internal junctions have electrical resistance values which are far too low to prevent substantial diffusional exchange during periods of prolonged use or storage.
A final consideration in the performance of reference electrodes is that their impedance (electrical resistance) should be as small as possible, since, as discussed in my co-pending application Ser. No. 230,457, "Noise Suppressing Bypass for Reference Electrode", the electrical noise susceptibility of an electrometer circuit is in most cases directly proportional to the impedance of the reference electrode.